The invention relates to landing gear for an aircraft. More particularly, the invention relates to landing gear of the type having a wheel truck formed by a bogie beam supporting forward and aft wheels at forward and aft ends thereof, a main strut pivotally connected to the bogie beam at a main pivot between the forward and aft wheels, and an additional mechanical linkage connected between the main strut and the bogie beam at a location spaced from the main pivot for controlling position of the bogie beam.
In most large commercial aircraft, the maximum rotation angle of the aircraft during takeoff and landing is limited by a minimum permissible clearance between a rear under portion of the fuselage and the ground. It is known that the takeoff and landing performance of a given aircraft can be enhanced by providing a longer main landing gear about which the aircraft rotates to achieve a nose-up attitude, thereby increasing the maximum rotation angle of the aircraft. However, one of the objectives of aircraft design is to configure the landing gear so that the aircraft fuselage is essentially horizontal during ground operations and has an appropriate sill height for ground servicing. The maximum sill height that is acceptable is dictated by the height of ground equipment that must interface with the aircraft, and thus is generally fixed. In many cases, the maximum allowable sill height is less than what would be desirable from an aircraft performance standpoint, and therefore, merely lengthening the landing gear is not a viable approach to achieving increased maximum rotation angle. Further, landing gear length must be minimized to keep weight to a minimum and to facilitate the stowing of the gear during flight, and hence a wholesale lengthening of the landing gear is undesirable.
In view of the above considerations, efforts have been made to develop variable-length landing gear capable of assuming a length that is suitable for stowing within the aircraft, and for ground operations while the aircraft is on the ground and stationary, and further capable of assuming a greater length during takeoff and landing operations. One such type of variable-length landing gear, to which the present invention relates, is the semi-levered landing gear (SLG). In a typical SLG, a wheel truck is formed by a bogie beam supporting forward and aft wheels at forward and aft ends thereof, and a main strut of conventional design is pivotally connected to the bogie beam at a main pivot between the forward and aft wheels. An additional mechanical linkage is connected at an upper end to the main strut and at a lower end to the bogie beam at an auxiliary pivot spaced from the main pivot for controlling positioning of the bogie beam. The additional mechanical linkage enables the bogie beam, under certain conditions, to pivot about the auxiliary pivot rather than the main pivot. In this manner, when the aircraft approaches the end of a takeoff roll and begins to rotate for liftoff, the bogie beam can be placed in a tilted orientation with the forward wheels off the ground with the aid of the additional mechanical linkage, which prevents the bogie beam from rotating to a horizontal orientation. With the wheel truck in this tilted position, the effective length of the landing gear is increased relative to its length when all wheels are on the ground. The aircraft can then rotate to a higher pitch attitude, with the same tail clearance, thus achieving improved takeoff performance.
Existing semi-levered landing gears can be unsatisfactory for various reasons.
In some types of SLG configurations, such as that disclosed in U.S. Pat. No. 4,892,270 to Derrien et al., the additional mechanical linkage comprises a passive torque link assembly whose only function is to lock up when the main strut and the bogie beam assume particular positions, namely, when the bogie beam is tilted and the main strut is relatively uncompressed as it is on initial touchdown and at liftoff. These types of SLG devices require an additional actuator or spring device for placing the bogie beam in the tilted position for landing. Where the means for tilting the bogie beam is a passive spring device as in the Derrien ""270 patent, stowing of the landing gear in the aircraft can be complicated by the lack of ability to reposition the bogie beam in a more-appropriate position for stowage.
One method that has been used to reposition the bogie for stowage with this type of SLG employs a shrink-link main strut that is operable to shorten as the landing gear is retracted into the wheel well, thereby changing the geometry of the SLG link and bogie so that the gear can be stowed. A disadvantage of this approach is that the shrink-link main strut is of considerably greater complexity and weight than a conventional main strut, thereby adding cost and weight to the aircraft.
Accordingly, some SLG configurations employ an active device connected between the main strut and the bogie beam for placing the bogie beam in a tilted position. For example, published UK Patent Application No. GB 2,101,542A by Putnam et al. discloses an aircraft undercarriage unit having a variable length oleopneumatic strut connected between the main strut and an aft end of the bogie beam. The variable length strut is hydraulically actuated to extend so as to tilt the bogie beam during takeoff. After takeoff, the variable length strut is contracted to position the bogie beam substantially horizontal to facilitate stowage of the gear. A major problem with Putnam""s landing gear design is that it is incapable of maintaining equal loading on all main gear wheels during braking at all aircraft weight and aerodynamic lift conditions, because the variable-length strut is always active to exert a force on the bogie tending to tilt the bogie, which occurs when the overall load on the landing gear drops to a sufficiently low level. The result is that Putnam""s landing gear would require larger brakes, and larger wheel wells to contain them, in order to assure adequate braking capacity during landing rollout or refused takeoff, thus incurring a significant penalty to the aircraft design in terms of weight and wheel well volume.
Another type of main landing gear is disclosed in UK Patent 1,510,554 by Faithfull. The Faithfull patent states as its object and advantage the capability of effectively lengthening the landing gear at touchdown to provide improved shock absorbing characteristics during landing at relatively high descent rates. The landing gear purportedly achieves this object by the use of an additional oil-filled cylinder, functioning only as a passive damper, pivotally attached to the front of the bogie beam and the upper stationary part of the main shock strut. In preparation for landing, the bogie is placed into a tilted position via a positioning device that is separate from the oil-filled cylinder. In this tilted position, the oil-filled cylinder is in a compressed condition. Upon touchdown and landing rollout, the bogie begins to rotate toward a horizontal position, thus causing the oil-filled cylinder to be extended until it reaches its maximum length. The maximum length of the oil-filled cylinder is such that the bogie cannot rotate to a fully horizontal position during the initial portion of the landing rollout, and hence the effective length of the landing gear is greater during this initial portion of the rollout.
Faithfull does not claim that his device is capable of providing improved takeoff performance through effective gear lengthening. Moreover, Faithfull""s device would prevent the most advantageous positioning of the bogie for stowage of the gear in the aircraft. In order to stow the landing gear in most aircraft, the bogie advantageously should be placed in an approximately horizontal position (on some large commercial aircraft, the bogie must rotate past horizontal into a pitch-down attitude of as much as 15 degrees) with the main strut fully extended, this orientation enabling the wheel well size to be kept to a minimum. However, Faithfull""s oil-filled cylinder has a maximum extension selected such that the bogie is tilted into a pitch-up attitude when the main strut is slightly compressed on landing. Thus, the oil-filled cylinder simply cannot extend sufficiently to position the bogie horizontal with the main strut fully extended. If the oil-filled cylinder disclosed in Faithfull were modified to provide sufficient stroke to accommodate the bogie stow position, it would be incapable of providing the semi-levered function on landing. Furthermore, if the stroke length were selected to provide effective semi-levered function on takeoff, then the bogie would assume a pitch-up attitude for stowage, which would require a very large wheel well. Thus, Faithfull""s device is incapable of simultaneously providing semi-levered function and enabling an optimum positioning of the bogie for stowage.
A main landing gear configuration disclosed in U.S. Pat. No. 4,749,152 is said to provide an effectively longer landing gear at takeoff, but requires a very complex main strut having multiple main strut cylinders, some with offset loading. This main strut would result in a very heavy landing gear relative to a conventional main strut. Additionally, the landing gear in the ""152 patent requires a shrink-link mechanism to reposition the bogie for stowage. Furthermore, the multiple-cylinder design results in sliding surfaces that cannot be inspected without major disassembly, thus increasing maintenance costs. Finally, another disadvantage of the gear design disclosed in the ""152 patent is that all of the purported functions of the gear, including semi-levered action at takeoff, absorption of energy at touchdown, equal wheel loading during ground roll, and bogie repositioning, are provided by the main strut. This may hamper the optimization of each of these functions because of space and geometry limitations of the design.
The present invention provides a semi-levered landing gear that, in preferred embodiments, is capable of eliminating the aforementioned shortcomings of the prior art. The landing gear includes a single auxiliary strut in conjunction with a main strut, which can be of conventional design, and a multiple-wheeled bogie. The auxiliary strut, in preferred embodiments of the invention, enables the landing gear to provide all of the desirable functions required of a main gear during aircraft operation, including:
(1) the ability to tilt the bogie to provide an effectively longer main landing gear during takeoff rotation and liftoff,
(2) the ability to reposition the bogie beam to an appropriate angle for stowing the landing gear;
(3) the ability to position the bogie beam to an appropriate pitch-up angle in preparation for landing after landing gear deployment, and thereby facilitate an early air-ground sensing upon initial ground contact of the aft bogie wheels;
(4) the ability to effectively decouple the auxiliary strut during static and ground-roll operations so as to facilitate equal loading of all main gear wheel and, accordingly, optimum braking ability;
(5) the ability to deactivate the functioning of the auxiliary strut that provides the semi-levered action when desired, such as during landing, so that the auxiliary strut acts as a damping device for partially absorbing touchdown loads such that the load transmitted to the aircraft is reduced.
To these ends, a semi-levered landing gear in accordance with a preferred embodiment of the invention comprises a wheel truck including a bogie beam and at least one forward wheel and at least one aft wheel rotatably supported by the bogie beam at forward and aft portions thereof, respectively, a main strut having an upper portion and a lower portion telescopingly connected to each other such that the main strut is extendable and compressible, the lower portion having a lower end pivotally connected to the bogie beam at a main pivot located between the forward and aft wheels, and an auxiliary strut having an upper end pivotally connected to the upper portion of the main strut and a lower end pivotally connected to the bogie beam at an auxiliary pivot longitudinally spaced from the main pivot. The auxiliary strut comprises a cylinder barrel having a closed end and an open end, a piston assembly slidably received through the open end of the cylinder barrel, and a lock-up device operable to permit extension of the piston assembly during a portion of a stroke thereof until the auxiliary strut reaches a predetermined lock-up length between a maximum length and a minimum length thereof. The lock-up device substantially prevents further extension of the piston assembly once the auxiliary strut reaches the predetermined lock-up length. The main strut and auxiliary strut are constructed and arranged relative to each other and the bogie beam such that, during takeoff as the main strut extends, the auxiliary strut becomes locked at the predetermined lock-up length before the main strut fully extends such that further extension of the main strut causes the bogie beam to pivot about the auxiliary pivot so as to tilt the bogie beam, whereby the landing gear is effectively lengthened. The main strut can be of conventional design; no shrink-link or other complex and heavy main strut is needed.
Preferably, the auxiliary strut is connected to a forward portion of the bogie beam. Thus, during a takeoff roll as the main strut extends, the auxiliary strut initially may extend but when it reaches its lock-up length it locks up. This causes the wheel truck to pivot into a nose-up attitude as the main strut further extends, forcing the aft wheels against the ground and thereby effectively lengthening the landing gear.
Preferably, the piston member of the auxiliary strut is actuatable by application of fluid pressure to cause retraction of the piston assembly to a predetermined landing approach length less than the predetermined lock-up length, whereby application of fluid pressure to the auxiliary strut prior to landing causes the wheel truck to be moved into a tilted position such that upon initial touchdown the aft wheel contacts the ground while the forward wheel is still above the ground. During landing, when the aft wheel makes initial contact with the ground, the auxiliary strut initially extends to allow the bogie beam to rotate slightly toward horizontal. When the auxiliary strut reaches the predetermined lock-up length, the lock-up device operates to stop further extension and prevent further rotation of the bogie beam.
In a preferred embodiment, the piston assembly divides the cylinder barrel into a pair of fluid chambers and defines a fluid path between the two fluid chambers such that extension and retraction of the piston assembly cause fluid to flow through the fluid path between the chambers. The lock-up device preferably comprises a lock-up valve disposed within the auxiliary strut and operable to maintain the fluid path open during a portion of an extension stroke of the piston assembly so as to permit extension of the piston assembly until the auxiliary strut reaches the predetermined lock-up length. The lock-up valve is operable to close the fluid path at the end of the portion of the stroke so as to substantially prevent further extension of the piston assembly and substantially lock the strut at the predetermined lock-up length.
In accordance with a further preferred embodiment of the invention, the piston assembly of the auxiliary strut includes a piston member actuatable by application of fluid pressure to cause extension of the piston assembly to the maximum length. The maximum length of the auxiliary strut is suitably chosen such that extension of the auxiliary strut to the maximum length after takeoff causes the wheel truck to be moved into a position having the bogie beam approximately horizontal to facilitate stowing the landing gear in the aircraft.
A still further preferred embodiment of the invention provides a dual-mode auxiliary strut having two different lock-up lengths, one optimized for takeoff and the other optimized for landing. In accordance with this embodiment, the lock-up valve is constructed such that application of a differential fluid pressure of one sense or an opposite sense across the lock-up valve causes the lock-up valve to be positioned in either an extended position or a retracted position and thereby increase or decrease, respectively, the length of the strut at which the lock-up valve closes the fluid path between the fluid chambers to lock up the strut. Thus, on landing approach, the lockup valve is actuated to move into the extended position so that the auxiliary strut undergoes a relatively longer extension stroke before locking up, thus enabling the bogie beam to rotate partially toward horizontal on initial touchdown so as to reduce transmission of touchdown loads to the aircraft. Conversely, on takeoff, the lock-up valve is actuated to move into the retracted position so that the auxiliary strut undergoes a relatively shorter extension stroke before locking up, thus facilitating a greater rotation angle of the aircraft.
Positioning of the lock-up valve is provided, in accordance with a preferred embodiment of the invention, by a fluid supply system connected to the auxiliary strut and operable to apply a differential fluid pressure of one sense or an opposite sense across the lock-up valve. Preferably, the fluid supply system includes a sensor operable to provide a signal when the aircraft touches down, and the fluid supply system is operable to reverse the sense of the differential fluid pressure applied across the lock-up valve upon a predetermined amount of time elapsing following receipt of the signal so as to retract the lock-up valve and place the auxiliary strut in a takeoff mode having a shorter lock-up length than a landing mode.
A preferred auxiliary strut has a piston assembly comprising a floating piston contained within the cylinder barrel and slidable therein along an axis of the cylinder barrel, the floating piston having an end wall that divides the cylinder barrel into the pair of fluid chambers and includes an aperture extending axially therethrough, and a main piston having a tubular portion slidably received through the open end of the cylinder barrel and through the aperture in the end wall of the floating piston such that the floating piston and main piston are slidable relative to each other. The main piston and floating piston cooperatively define the fluid path between the two fluid chambers, and the lock-up valve is arranged with respect to the main piston and floating piston to close the fluid path when the piston assembly is extended to the predetermined lock-up length. In one preferred embodiment, the tubular portion of the main piston defines snubbing holes therethrough providing the fluid path between the two fluid chambers, and the lock-up valve is arranged to open and close the snubbing holes upon movement of the main piston relative to the lock-up valve. Preferably, the lock-up valve includes a valve member slidably disposed in the tubular portion of the main piston. The valve member is positioned relative to the snubbing holes such that extension of the main piston causes the valve member to close the snubbing holes.
The floating piston and main piston advantageously include cooperative engagement portions that coact upon retraction of the floating piston in the cylinder barrel such that the floating piston retracts the main piston. In one embodiment, the engagement portions comprise the end wall of the floating piston and an annular flange portion attached to the tubular portion of the main piston and arranged to be abutted by the end wall of the floating piston when the floating piston is retracted. Retraction of the floating piston thus retracts the main piston so as to reduce the length of the auxiliary strut. This enables the bogie beam to be placed in a tilted position in preparation for landing.
Actuation of the floating piston to retract the main piston is accomplished by application of fluid pressure across the floating piston. To this end, the floating piston preferably includes a generally cylindrical portion connected to the end wall of the floating piston and arranged within the cylinder barrel such that an annular floating piston control chamber is defined between the cylindrical portion of the floating piston and an inner wall of the cylinder barrel. The auxiliary strut includes a fluid passage extending into the floating piston control chamber and adapted to be connected to a fluid source operable to selectively vary the pressure of fluid supplied through the fluid passage into the floating piston control chamber for controlling movement of the floating piston.
To facilitate positioning the bogie beam horizontally for gear stowage, the auxiliary strut preferably is actuatable to extend the floating piston and main piston to a maximum length. Further, the floating piston preferably is both retractable and extendable by fluid pressure. This is accomplished by providing a fluid passage extending into one of the fluid chambers of the strut and adapted to be connected to a fluid source. A fluid supply system is connected to the fluid passages that extend into the floating piston control chamber and the one of the fluid chambers of the strut. The fluid supply system is operable to apply a differential fluid pressure of one sense between the floating piston control chamber and the one of the fluid chambers so as to retract the floating piston which in turn retracts the main piston, and is operable to apply a differential fluid pressure of an opposite sense between the floating piston control chamber and the one of the fluid chambers so as to extend the floating piston and the main piston. Thus, the auxiliary strut preferably provides both a capability of tilting the bogie beam for landing and a capability of placing the bogie beam in a horizontal position for gear stowage.